MOSCOW — Russia said Thursday it will expel four British diplomats and suspend counterterrorism cooperation with London, the latest move in a mounting confrontation over the radiation poisoning death of former KGB officer Alexander Litvinenko.

Britain had announced Monday the expulsion of four Russian diplomats and restrictions on visas issued to Russian government officials after Moscow refused to extradite Andrei Lugovoi, accused of killing Litvinenko in London last November.

A spokesman for Prime Minister Gordon Brown called the Russian move “completely unjustified.”

A new lowThe dispute marks a new low in relations between Moscow and London, which had already been troubled by Russia’s opposition to the war in Iraq, Britain’s refusal to extradite exiled tycoon Boris Berezovsky to face embezzlement charges, and by Kremlin allegations last year of spying by British diplomats.

Foreign Ministry spokesman Mikhail Kamynin announced the expulsions after summoning British Ambassador Anthony Brenton to the ministry and informing him.

Kamynin described Russia’s response as “targeted, balanced and the minimum necessary.” He contended that Russia was forced to respond, saying Britain had made a “conscious choice of worsening relations with our country.”

Brown spokesman Michael Ellam said Britain viewed Russia’s action as “completely unjustified, and we will continue to take this matter forward with the international community over the next weeks.”

“We obviously believe that the decision to expel four embassy staff is completely unjustified and we will be doing everything to ensure that they and their families are properly looked after,” he said.

Rice backs British stanceSecretary of State Condoleezza Rice called on Moscow to heed British demands and extradite Lugovoi — the first time America’s senior diplomat has weighed in on the dispute.

“This is an issue of rule of law to our minds, not an issue of politics,” Rice said at a news conference in Portugal, where she was attending a conference on Middle East peace. “It is a matter of Russia cooperating fully in what is simply an effort to solve what was a very terrible crime committed on British soil.”

Russia says such extraditions are prohibited by its constitution and characterized Britain’s demand as an attempt to interfere in Moscow’s internal affairs.

“We are disappointed that the Russian government should have signaled no new cooperation in the extradition of Mr. Andrei Lugovoi for the alleged murder of Alexander Litvinenko,” Miliband said.

“We are, however, much heartened that over the last 36 hours across the international community, European countries, the EU as a whole and the United States should have put out such positive statements about the need to defend the integrity of the British judicial system, and that is something that we shall be taking forward with the international community over the next few days and weeks,” Miliband said.

Litvinenko, a fierce Kremlin critic, died Nov. 23 after ingesting radioactive polonium-210. From his deathbed, he said Russian President Vladimir Putin was behind his poisoning.

Moscow denounces claims of official involvementA letter from Russia’s ambassador in London denounced claims of Kremlin involvement in Litvinenko’s murder.

“It is preposterous to assert that the killing of Alexander Litvinenko ‘appears to have the clear backing, if not the active assistance, of the Russian government,”’ Ambassador Yuri Fedotov wrote in a letter to The Times, responding to an editorial published Tuesday.

Fedotov said there is nothing sinister in Russia’s refusal to hand over Lugovoi, and reaffirmed Russia’s offer to put him on trial at home if British authorities provide enough evidence.

Fedotov, who has been uncharacteristically visible in British media this week, said that how far the standoff goes depends on the “political will” of the British government.

“The Russian government values its relations with the U.K. and respects its laws and constitutional arrangement,” Fedotov wrote. “A close relationship, of course, requires that the British government does the same.”

Plot to kill Russian exile tycoon?British police said Wednesday that it had apprehended and deported a suspected Russian assassin who was reportedly planning to murder Berezovsky in June. The tycoon accused the Kremlin of being behind that plot.

Kamynin also said Russia would stop issuing visas to British officials and seeking British visas for Russian officials until London provides more information on the restrictions it has imposed.

“Until the new procedure is explained, Russian officials will not request British visas. And analogous requests by British officials will not be considered,” he said.

He did not elaborate, and the extent of current cooperation — with ties already damaged by Russian intelligence services’ accusations of British spying — was unclear.

Natalia Leshchenko, an analyst at the Global Insight think tank, said on BBC TV that Britain and Russia do cooperate against terror, but suggested the suspension was mainly meant to tarnish Brown’s new government in the eyes of its own people.

“The cooperation itself is there and we can also say it can be exaggerated if needed to show that Gordon Brown acts against the British people. At least that’s what they are saying to the Russian public at the moment,” Leshchenko said.

Kamynin said the interests of tourists and businessmen would not be hurt. He said that on visa issues, Russia would mirror Britain’s actions from now on.